Mobile or portable radio terminals in cellular networks permit users of such radio terminals to move from place to place within a geographic area and maintain or establish a communication channel with base stations in the network. The base stations are typically connected to an existing telephone network, thereby providing access to a larger telecommunication system.
Situations often arise where it is desirable to determine the location of a mobile or portable radio terminal. Such situations may arise, for example, in the event of a medical or safety emergency so that help may be dispatched. Similarly, it may be desirable to track a radio terminal as it moves within a particular geographic area for the purpose of tracking assets or people that are travelling with the radio terminal.
Several techniques are known for locating a mobile or portable radio terminal. Loran-C receivers, for example, use a radio pulsed navigation system. Another proposed technique would use the Global Positioning System (GPS) in conjunction with a cellular telephone network. The GPS, however, is a satellite based navigation and positioning system and does not perform well inside buildings. Furthermore, the system would require that a user have both a GPS receiver as well as cellular telephone service.
Another technique for locating a mobile or portable radio terminal is disclosed in U.S. Pat. No. 5,293,645. The disclosed method involves compiling relative propagation delays for synchronized timing signals transmitted from a plurality of base stations in a cellular network and received by the radio terminal to be located. The radio terminal then transmits the compiled information to a base station or other unit in the network where the location of the radio terminal is identified. One feature of the disclosure of the aforementioned patent is that in order to identify the location of the radio terminal, the radio terminal must transmit information to a unit in the network that is external to the radio terminal. Furthermore, according to the disclosure of the aforementioned patent, the user of the radio terminal does not have immediate access to its location.